Although the nondescript, black, child-size backpacks sold by the Massachusetts body armor company Bullet Blocker look like regular backpacks, a sheet of body armor is sewn inside each bag as "another protective layer."

Elmar Uy, chief operating officer at Bullet Blocker, noticed his sales numbers were up "tenfold" on Friday, the day of the massacre, but said he didn't understand why until he turned on the news.

"When word gets out there is an option, not a complete solution, to protect their kids, parents go and seek it," he said.

Amendment II, a Utah-based company that manufactures lightweight armor for law enforcement and the military, began inserting their technology into kids' backpacks six months ago after they received several custom orders, said Derek Williams, the company's president.

"We would sell a few here and there, and it was very much a niche item. But following Friday, our sales have gone up over 500 percent in childrens' armor products," Williams said.

The backpacks aren't meant to be worn during an active shooter situation, but rather as a shield "to cover their head and vital areas," Uy said.

Uy and Williams, who are both fathers, recognize that bulletproof backpacks and the inserts their companies sell aren't a solution to surviving a school shooting.

"There is only so much you can do," Williams said. "The bottom line is, having some armor is better than none. I don't want my kids to be unprotected in schools, which are becoming increasingly violent."

Amendment II plans to donate a portion of their sales to the families of Sandy Hook victims, Williams said.

"On Friday my business partners and I were in tears along with everyone else. We're all fathers," he said. "We can't do much except do what we can and what we're good at, which is making good body armor."